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A hungry falcon soars high above Earth. Its sharp eyes scan the ground. Suddenly, it spies something moving in the grass. The falcon dives toward it. Far below, a gray field mouse scurries through the grass. Its dark, beady eyes search constantly for danger. With eyes on either side of its head, the mouse can see almost everything around it.

Will the mouse see the falcon in time to escape? Or, will the speedy falcon catch the prey it spied from far above? Whatever happens, one thing is clear: Without eyes, neither animal has a good chance.

Why? Eyes help many animals make sense of the world around them—and survive. Eyes can guide the falcon to dinner or help the mouse see a perfect place to hide.

Animal eyes come in many different shapes, sizes, colors, and even numbers. Yet they do the same job. They all catch light. With help from the brain, eyes turn light into sight.

Eyes work in the same way for people. Look at this page. You may think you see words and pictures. Believe it or not, you don't. All you see is light bouncing off the page. How is this possible? The secret is in the rules of light.

Light is a form of energy, like heat or sound. It can come from a natural source, like the sun, or artificial sources, like a lamp or a flashlight.

Light is the fastest known thing. It travels in waves and in nearly straight lines. In air, it can speed 299,700 kilometers (186,200 miles) per second. It can race from the sun to Earth in just over eight minutes! Light doesn't always travel so fast. For example, water or glass can slow light down, but just a bit.

Light may seem to break all driving speed laws. Yet there are certain rules it always follows. Light reflects, or bounces off objects. It also refracts, or bends. And it can be absorbed, or soaked up, by objects. These rules of light affect what, and how, we see.

Imagine this scene: You're at your computer happily reading this article. Light from your desk lamp scatters in all directions.

Light hits the screen. Some bounces off the screen, or reflects. It changes direction. It's a little like how sound bounces off a wall. Now some of this reflected light is traveling right toward your face. Don't duck! For you to see the words, some of this light has to enter your eyes. Objects become visible when light bounces off them.

Your eyes are light catchers. Yet it takes more than catching light to see an image. Your eyes also have to bend light. Here's how. First, light hits your cornea. That's the clear covering on the front of your eyeball. The cornea refracts, or bends, light.

Is your cornea super strong? No! Think about how light travels more slowly through water. The same thing happens in your cornea. As light passes through the cornea, it slows down. That makes the light change direction, or bend.

Next, light enters your pupil, the dark center part of your eye. It passes through your lens. The lens bends light, too. What's the big deal about bending light? That's how your eyes focus, or aim the light to make a clear image.

The image appears on your retina at the back of your eyeball. It's like a movie. Playing Today at a Theater in Your Eye: Explorer magazine! There's only one problem. The image is upside down. Luckily, your brain flips the image right side up. That's pretty smart!

At night, your eyes work to catch more light. Look at the colored ring around your pupil. That's the iris. It's a muscle that changes the size of your pupil. In dim light, the iris makes your pupil bigger to let in more light. In bright light, the iris makes your pupil smaller. That lets less light enter your eye.

Special parts in your retina also help night sight. They are called rods. Rods tell your brain that light is hitting your retina. Rods work best in dim light. They help you see in black, white, and gray. Each eye has 125 million rods!

Even so, it's hard for people to see well in the dark. It's easier for nocturnal animals. Many have extra big eyes for the size of their heads. Some even have an extra part in their eyes.

Think of a cat at night. Its eyes seem to glow. The cat has a part behind its retina called a tapetum. It acts like a mirror. The cat's eye catches light once on the way in, and a second time when light reflects off the tapetum. The cat's eyes get two chances to catch light.

In dim light, you see mostly shades of gray. It takes bright light to see many colors. In the daytime, light may look white but it's really made up of many different colors.

Remember the light waves? Each wavelength has a different color. Check out the picture of a prism (above). It shows white light entering. Then it shows how clear glass in a prism bends and separates the light waves. So you can see a rainbow of colors!

When light hits an object, some colors are absorbed. Others are reflected. The colors absorbed depend on what the object is made of.

Look at a leaf. Does it look green? That's because it's soaking in a lot of the other colors. The leaf reflects more green light than any other color. That's why we see the leaf as green.

It takes more than light to see color. It takes cones. Like rods, cones are special parts in the retina. Each human eye has 7 million cones. Yet they aren't all the same. People have three kinds of cones. Each type senses different wavelengths of light. The cones are most sensitive to red, green, and blue.

When these light waves hit the cones, they cause a reaction. The cones then send color messages to the brain. The brain mixes the colors together. That's why we can see more colors than red, green, and blue.

Some animals have different kinds of cones than people have. Scientists think these animals see in ways that humans can't. For example, compared to people, a bee may get a different view of a yellow dandelion. It may see patterns made by ultraviolet light—light waves humans can't see. These patterns help the bee zero in on a good landing spot.

Other animals don't have any cones at all. They can only see in black, white, and gray. That's how the giant squid sees its world. Yet it has more rods than many creatures. The extra rods help the giant squid's eyes sense as much light as possible in the deep, dark ocean.

No matter whose face they're on, all eyes have a lot in common. Some animals have pupils with an odd shape. Others have special ways to move their eyes or to protect them.

Many animals have two eyes. Others have dozens. Animals can have eyes in different places on their faces—or even their bodies! Why are these eyes so different? It's all about catching the light an animal needs to survive. Think of the falcon. Its eyes are really big for the size of its head. So are its retinas. That lets the falcon's eyes sense more light.

The falcon's eyes also are on the front of its head. They face forward. That lets both eyes see much of the same view. This is called binocular vision. The falcon gets information about the same view two times, once from each eye. This helps the falcon figure out distances. How far will it have to dive to catch that mouse?

Other animals, like the mouse, have eyes on the sides of their head. Each eye sees a different view. This is called monocular vision. These animals can see in front, above, below, beside, and even behind their bodies. That helps them try to keep one step ahead of predators.

A chameleon's eyes can even move in different directions at the same time. One eye looks forward. The other looks backward to see if anything is sneaking up from behind!

Maybe the most sneak-proof eyes are the fiddler crab's. Its eyes stick up high above its body. This lets the crab see in many different directions at once. For a really good view, try checking out the world through dozens of eyes. That's what the box jellyfish does. It has 24 eyes dangling from its body! Having so many eyes may help the box jellyfish skim the ocean floor without bumping into rocks.

Insects may have the wildest eyes of all. Just look at the bulging eyes of the damselfly on the previous screen. They take up most of its head! These eyes are called compound eyes. Each one has many tiny lenses. Each lens catches light and creates an image. The bug's brain puts all the images together. It's a little like putting together a giant puzzle.

A fly's eye has 3,000 lenses. A bee's eye has more than 5,000. And the dragonfly has the most of all: a whopping 30,000! These super eyes don't give insects super sharp vision. In fact, their eyes don't focus the light very well. But these oddball eyes are super motion detectors.

How? Having thousands of lenses helps an insect see even the smallest flickers of movement. That's one reason why it's so hard to catch pesky flies. They see the swatter coming and fly off in a flash.

Engineers have set their sights on cool animal eyes. They study how all these eyes work. It's giving them ideas for new inventions.

Understanding how eyes capture light may lead to designs for cameras that take pictures inside the human body or to better ways to help planes land safely. Someday, scientists might even be able to make an artificial eye that works just like the real thing. Eyes just might help us see a better, brighter future.

Article by Leslie Hall. Top-of-page illustration from © Ivanagott/Shutterstock. "Seeing Eye to Eye" appears on page 2 of the September 2009 issue.